1. Field of the Invention
The present invention relates to a pump drive structure for a two-cycle engine.
2. Background Art
Japanese Utility Model Publication No. Hei 2-13707 discloses a pump drive structure in which a balancer shaft is provided in parallel to and in front of a crank shaft of a two-cycle engine and is driven by a balancer drive gear, and a water pump is disposed concentrically with the balancer shaft is driven by the balancer shaft.
Japanese Patent Laid-open No. Hei 4-112924 discloses a pump drive structure in which a water pump and an oil pump having pump shafts in parallel to a crank shaft of an engine are disposed in front of a crank case in such a manner so as to be positioned to the right and left, and the pump shafts are rotated by a balancer shaft disposed in parallel to the crank shaft and rotated by the crank shaft.
Incidentally, since a two-cycle engine primarily compresses a fuel-air mixture in a crank chamber, a balancer shaft must be partitioned from the crank chamber, and further, when the balancer shaft is close to the crank shaft for making compact the engine, a balancer weight is distributed on both sides of the crank weight.
In the case of adopting a transmission type in which a drive side pulley of a V-belt type transmission is mounted on one end of a crank shaft, if a balancer drive gear is provided on a crank shaft on the drive side pulley side, there are required a wall for partitioning the crank chamber from a balancer drive gear chamber for lubrication of the gear, a seal for the wall portion, and a wall for partitioning the balancer drive gear chamber from the atmospheric air side for supporting the drive side pulley, as a result of which the drive side pulley must be disposed separately from the crank chamber by the presence of these walls, seal, and the thickness of the balancer drive gear.
Consequently, since not only the vehicular width is increased but also the crank shaft is made longer, the crank shaft must be increased in diameter in order to withstand a belt tension of the V-belt transmission, so that the weight is increased.
On the other hand, by driving a water pump and an oil pump using a balancer shaft, the drive structure for these accessories is simplified. Accordingly, in the case where the pump is driven by making use of the balancer shaft and simultaneously the V-belt type transmission is connected to a crank shaft, it is required not to increase the width and the weight of the body.